Fifty years ago, biochemists described cells as small vessels that contain a complex mixture of chemical species undergoing reactions through diffusion and random collision. This description was satisfactory inasmuch as the intricate pathways of metabolism and, later, the basic mechanisms of gene regulation and signal transduction were still being unraveled. Gradually, and in part as a result of the parallel growth in the structural understanding of the molecular components of the cell, together with the development of single molecule manipulation methods, scientists have become increasingly aware that the cell resembles more a mechanical factory in which many of the processes are performed by specialized machinery whose behavior is essentially mechanical in nature. Biochemical processes as diverse as the elastic response of DNA, protein-induced DNA bending, chromosomal segregation, replication, transcription, translation, protein translocation across membranes, catalyzed protein and nucleic acid folding and unfolding, and even the ubiquitous processes of induced-fit molecular recognition, are all examples in which forces and torques develop in molecules as they move along their reaction coordinates. They are thus, amenable of study by direct manipulation methods. Methods of single-molecule manipulation (such as optical tweezers or atomic force micrsocopy) are being used today to: a) measure directly the forces holding molecular structures together, b) determine the stresses and strains generated in the course of chemical and biochemical reactions, c) exert external forces or torques to alter the extent and fate of these reactions, and d) reveal the rules that govern the interconversion of mechanical and chemical forces in these processes. This area of research can be rightly called mechanochemistry.
